The application of a photodiode array (PDA) as the measuring photometric device for a spectrophotometer is normally to deliver as ordinate an output curve of intensity, or a related quantity such as percent transmittance or absorbance, plotted against wavelength as abscissa. This curve has been observed to be vitiated by the appearance of periodic ripples of small amplitude superimposed on the true curve. The periodicity of these ripples is typically a function of wavelength with the ripple peaks occurring at shorter wavelength intervals when measuring in the ultraviolet and increasing in spacing gradually as the wavelength increases to the infrared end of the curve. Investigation indicates this to be an interference phenomenon associated with the construction of the PDA. It is important to eliminate or minimize this objectionable artifact.
As a basis of this invention an optical means is disclosed which acts on the converging spectral beam falling on the receptor face of the PDA redirecting different parts of this beam in such a manner as to suppress the action causing the artifact.
It is an object of this invention to suppress an interference-like artifact on the output data curve of an array spectrophotometer.
It is a further object of this invention to suppress such artifacts without introduction of errors on the output data curve due to minute motions or position changes of the PDA or parts of the spectrophotometer monochromator.
It is further object of this invention that the means used for the foregoing objectives be an external addition to the PDA requiring no modification of the commercial PDA construction.
It is a further objective that the means used to suppress the artifact previously mentioned treats each array detecting element independently so that no stray light error or loss of resolution is incurred by light scatter or dispersion.
It is also an objective that no impairment of wavelength accuracy shall be incurred.
It is a further objective that no substantial attenuation of the intensity level of the output signal shall result.